There are many uses for a device that can sense orientation relative to the Earth's gravity vector. Such information is useful, for example for navigation equipment and for stabilizing apparatus, and the like. Another important use is in instruments and devices that sense the Earth's magnetic field, and use that information for such as navigational purposes.
Inclination is important for instruments that sense the Earth's magnetic field, because the orientation of a sensing device, typically called a magnetometer, greatly effects reading and determination of the field magnitude and direction.
There are a number of devices known for measuring inclination, taking advantage of the Earth's gravity vector as an absolute standard. One such device is known in the art as a dual axis inclinometer. There are two basic types of such inclinometers. One is a capacitance type that uses a fluid in an annular container having electrodes positioned on opposite walls of the annulus. Typically one wall of the annulus serves as a common electrode, and the opposite wall has four separate electrodes arranged at ninety-degree intervals, so there are two electrode pairs on horizontal axes at right angles to one another and to the axis of the annulus as well.
A second sort has typically a sealed cylinder partially filled with electrolytic fluid, and five electrodes that penetrate the floor of the cylinder. Four of the electrodes are arranged in a pattern at the corners of a square, with the fifth at the center of the square. This is a resistance inclinometer, as opposed to a capacitance type inclinometer. In the present invention, the electrolytic resistance type inclinometer is of particular interest.
As described above, in a liquid-filled electrode-type inclinometer, it is the relative level of fluid between electrodes that determines the measurable electrical resistance between electrodes. The arrangement of the five electrodes allows electrical measurements between any two to be used to determine the tilt of the container into which the electrodes protrude.
There are serious difficulties associated with both the capacitance measurement and the resistance measurement approaches to determining inclination. For example, in the capacitance measurement approach, the fluid itself has a high dielectric strength, and a pair of electrodes empty of fluid still shows high capacitance. The result is that the difference in capacitance for a pair of electrodes filled with fluid to the maximum level, and practically devoid of fluid at the other extreme, is quite small, so the resolution of such an instrument is severely limited, and relative errors may be quite high. Circuitry to discriminate in such a tight resolution band is also generally more expensive to build and operate.
There is no equivalent resolution problem in the resistance approach. In this approach, the fluid is electrolytic, such as a salt solution, and is relatively electrically conductive compared to the dielectric approach. Circuitry connected to the electrodes in a resistance-type inclinometer is implemented, as stated above, to measure resistance between electrodes, and to use the resistance measurements to determine tilt of the electrode container.
For the resistance inclinometers the resistance between electrodes varies over a very broad range of resistance, essentially between a relatively low resistance and infinite resistance. This is enough to provide fine resolution. The electrolytic fluid, however, is very sensitive and fragile. Due to the presence of electrodes and the ionic nature of the fluid, a net electrical current in one direction will change the composition of the fluid drastically. In a typical case a net dc component across an electrode of 1 .mu.Amp for 10 seconds will render the fluid useless, effectively destroying the inclinometer element of the instrument.
To counter the extreme electrical delicacy of electrolytic inclinometers to net dc current, manufacturers lave developed circuitry to test the resistance between electrodes with an alternating current having as nearly as possible a 50% duty cycle. That is, the current flows in one direction as nearly as possible with the same average amplitude and time as in the opposite direction.
One way manufacturers have attempted to solve the problem is by providing square waves for testing using very accurate generating circuitry. To do this, however, requires careful selection of components, such as resistors, capacitors, inductors, and the like for the square wave generator. As there is a practical limit to the repeatability of characteristics between manufactured components, for example, the components may often have to be selected and tested by hand to provide matching values to build such accurate devices.
What is clearly needed is a circuit for utilizing a resistance-type inclinometer device in a manner that avoids net dc current between electrodes, and therefore avoids deteriorating electrolytic fluid, without requiring expensive, hand-selected and tested components, and does so with circuitry of low complexity and cost relative to conventional solutions.